MAC address

i am going to assume two things 1) you are running windows ans 2) you have there ip address.

go into your cmd and type "nbtstat -a XXX.XXX.XXX.XXX"
where XXX.XXX.XXX.XXX is the ip of the computer that you want to know the ip of. you should get some info and at the bottom is should say MAC Address = then the mac address.